Electroluminescent devices that employ an organic material for light emission are described in PCT/WO90/13148 and U.S. Pat. No. 4,539,507, the contents of both of which are incorporated herein by reference. The basic structure of these devices is a light-emissive organic layer, for instance a film of a poly(p-phenylenevinylene (“PPV”), sandwiched between two electrodes. One of the electrodes (the cathode) injects negative charge carriers (electrons) and the other electrode (the anode) injects positive charge carriers (holes). The electrons and holes combine in the organic layer generating photons. In PCT/WO90/13148 the organic light-emissive material is a polymer. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinolino) aluminium (“Alq”). In a practical device, one of the electrodes is typically transparent, to allow the photons to escape the device.
These devices have great potential for displays. However, there are several significant problems. One is to make the device efficient, particularly as measured by its power efficiency and its external efficiency. Another is to reduce the voltage at which peak efficiency is obtained.
As a preliminary point, it should be noted that the values stated here for energy levels, workfunctions etc. are generally illustrative rather than absolute. The workfunction of ITO can vary widely. Numbers quoted in the literature suggest a range between 4 and 5.2 eV. The 4.8 eV value used here serves as an illustrative rather than an absolute value. The applicant has carried out Kelvin probe measurements which suggest that 4.8 eV is a reasonable value. However, it is well known that the actual value can depend on ITO deposition process and history. For organic semiconductors important characteristics are the binding energies, measured with respect to the vacuum level of the electronic energy levels, particularly the “highest occupied molecular orbital” (“HOMO”) and “lowest unoccupied molecular orbital” (“LUMO”) levels. These can be estimated from measurements of photoemission and particularly measurements of the electrochemical potentials for oxidation and reduction. It is well understood in the field that such energies are affected by a number of factors, such as the local environment near an interface, and is the point on the curve (peak) from which the value is determined—e.g. peak, peak base, half-way point—so the use of such values is indicative rather than quantitative. However, the relative values are significant.
FIG. 1a shows a cross section of a typical device for emitting green light. FIG. 1b shows the energy levels across the device. The anode 1 is a layer of transparent indium-tin oxide (“ITO”) with a workfunction of 4.8 eV. The cathode 2 is a LiAl layer of with a workfunction of 2.4 eV. Between the electrodes is a light-emissive layer 3 of PPV, having a LUMO energy level 5 at around 2.7 eV and a HOMO energy level 6 at around 5.2 eV. Holes and electrons that are injected into the device recombine radiatively in the PPV layer. A helpful but not essential feature of the device is the hole transport layer 4 of doped polyethylene dioxythiophene (“PEDOT”) (see EP 0 686 662 and Bayer AG's Provisional Product Information Sheet for Trial Product AI 4071). This provides an intermediate energy level at 4.8 eV, which helps the holes injected from the ITO to reach the HOMO level in the PPV.
Other organic light-emissive materials, having different optical gaps, can take the place of the PPV in order to generate light of other colours. However, at larger optical gaps, towards the blue end of the visible spectrum, the HOMO level is generally well below the corresponding energy level of the ITO. This makes it difficult to inject holes into the emissive layer, i.e. high electric fields are required in order to encourage holes to inject into the semiconductor layer. One solution to this problem would be to choose another material for the anode, but it is difficult to find a preferable alternative because ITO has good transparency, low sheet resistance and established processing routes. Another solution is to add further hole transport layers, so as to provide a series of intermediate energy steps between the anode and the emissive layer. However, where the layers are deposited from solution it is difficult to avoid one layer being disrupted when the next is deposited, and problems can arise with voids or material trapped between the increased number of inter-layer boundaries.
Considerable advantages can be had from using a plurality of organic semiconductors within a diode structure; critical to the functioning of such structures is the nature of the interface electronic structure between any two components in contact with one another. A common starting point for such descriptions is that well-known for heterojunctions formed in epitaxially-grown III-V semiconductors. Heterojunctions are classified into classes which include: type I, in which the LUMO and HOMO levels of one material (material A) lie within the LUMO-HOMO energy gap of the second material (material B), as illustrated in FIG. 2a, and type II, in which the minimum energy difference between the highest HOMO state and the lowest LUMO state is between levels on different sides of the heterojunction, as illustrated in FIG. 2b. It is generally considered that an electron-hole pair that is in the immediate vicinity of such heterojunctions will arrange so that the electron occupies the lowest LUMO level, and the hole occupies the highest HOMO level. Thus, the electron and hole are present on the same side of the junction for a type I heterojunction, but are separated for the type II heterojunction. An important consequence of this is that electron-hole capture and subsequent light emission is expected for type I but not for type II heterojunctions.
There have been some attempts to combine components in blue-emissive layers. In “Highly Efficient Blue Electroluminescence from a Distyrylarylene Emitting Layer with a new Dopant”, Hosokawa et al., Appl. Phys. Lett. 67 (26), 25 Dec. 1995, pp 3853-5 a small molecule device has an emissive layer in which DPVBi is blended with BCzVB or BczVBi. The dopants have a slightly smaller bandgap and a displaced HOMO position compared to the host material. The observed light emission is only from the dopant. This is explained by the authors as arising from Förster energy transfer due to the smaller energy of an exciton on the dopant molecules. “Efficient Blue-Light Emitting Devices from Conjugated Polymer Blends”, Birgerson et al., Adv. Mater. 1996, 8, No. 12, pp 982-5 describes a blue-light emitting device which employs conjugated polymer blends. The emissive layer of the device consists of a blend of PDHPT with PDPP. These materials form a type I semiconductor interface (see FIG. 2a), so light emission is from the PDHPT alone. The paper emphasises that “it is a necessary but not sufficient requirement that the HOMO-LUMO gap of the light-emitting (guest) polymer be smaller than that of the host polymer. An additional condition is that . . . the HOMO energy level of the guest polymer must be at a lower binding energy than that of the host polymer, and the LUMO energy level of the guest polymer must be at a higher binding energy than that of the host polymer”. Other devices having type I interfaces at the emissive layer are described in EP 0 532 798 A1 (Mori et al.) and U.S. Pat. No. 5,378,519 (Kikuchi et al.).
Two-layer EL devices which exploit the high electron affinity of cyano-derivatives of PPV have shown high efficiencies, as described in U.S. Pat. No. 5,514,878. However, when mixtures are formed with CN-PPV and the soluble PPV, MEH-PPV, as described in “Efficient Photodiodes from Interpenetrating Polymer Networks”, J J M Halls et al., Nature, Vol. 376, 10 Aug. 1995, pp 498-500 and U.S. Pat. No. 5,670,791, strong quenching of luminescence is observed.
“Oxadiazole-Containing Conjugated Polymers for Light-Emitting Diodes”, Peng et al., Adv. Mater. 1998, 10, No. 9 describes a light-emitting device in which the emissive layer comprises an oxadiazole-containing PPV polymer. The oxadiazole is present to aid electron transport. It is noted that “the PPV segment can function as both the hole transporter and the emitter”.
In “Efficient Blue LEDs from a Partially Conjugated Si-Containing PPV Copolymer in a Double-Layer Configuration”, Garten et al., Adv. Mater., 1997, 9, No. 2, pp 127-131 a light-emissive device has an emissive layer in which Si-PPV is diluted with PVK to reduce aggregation. The photoluminescent efficiency of the device is observed to increase when aggregation is reduced.
“Blue Light-Emitting Devices Based on Novel Polymer Blends”, Cimrová et al., Adv. Mater. 1998, 10, No. 9 describes light-emitting devices whose emitting layers comprise a blend of two polymers having “almost identical HOMO levels”.